The development of new techniques for the utilization of non-polluting, renewable sources of energy is of paramount interest to meet the energy demands of the future. Solar energy is among the energy sources of greatest interest because of its non-polluting nature and its abundant, non-diminishing availability. One approach to the utilization of solar energy involves the conversion of solar energy into electricity by means of the photovoltaic effect upon the absorption of sunlight by solar cells.
Silicon solar cells, the most commonly employed devices based on the photovoltaic effect, have been utilized reliably in spacecraft applications for many years. Crystals of ultra-high purity silicon are also important for use as semiconductors in the electronics industry. However, the costs associated with the production of such high purity, high-perfection crystals for these applications and for industrial and commercial applications in general, are often not economically feasible.
High purity silicon is generally prepared by procedures involving the conversion of metallurgical grade silicon to silane or a halosilane, which is subsequently purified and then reduced to produce polycrystalline, semiconductor grade silicon from which single crystals can be grown. One such process to upgrade metallurgical grade silicon to the ultra-high purity silicon needed for either solar cell or semi-conductor usage is disclosed in U.S. Pat. No. 4,340,574 to Coleman. This process includes reaction of metallurgical grade silicon with silicon tetrachloride and hydrogen to yield an intermediate trichlorosilane and silicon tetrachloride feed stream. The intermediate feed stream is further processed in a redistribution reactor and distillation column section whereby the trichlorosilane is upgraded and separated into an ultra-high purity silane product and recycle silicon tetrachloride. The high purity silane is pyrolyzed to produce the ultra-high purity silicon product and hydrogen by-product.
It is well known in the prior art to reduce a halosilane product such as trichlorosilane to polycrystalline silicon in a reactor having carrier rods electrically heated to about 1100.degree. C. on which reactant gases precipitate silicon. This process, commonly known as the Siemens process, is described in U.S. Pat. No. 3,979,490 to Dietze et al. This batch-type process, which is now highly commercialized, possesses high material costs and power requirements.
Another method for obtaining high purity silicon from silane or a halosilane is by pyrolysis in a fluidized bed. Silicon seed particles are suspended in a fluidizing gas stream into which silane or a halosilane is injected through some type of gas distribution apparatus. Process conditions are desirably maintained so that the silane decomposes to silicon on the surface of the seed particles in the fluidized bed. By this process seed particles grow by silicon deposition thereon and drop out of the reaction zone falling through the fluidized gas stream into a collector such as a boot-type separator. Hydrogen, which may be formed as a by-product from the silane decomposition, can be removed separately overhead from the reaction zone. When chlorosilanes are decomposed the by-product gas generally comprises hydrogen chloride.
The pyrolysis of silane or a halosilane may be achieved by capacitive heating of the fluidized bed reaction zone, as disclosed in U.S. Pat. No. 4,292,344 to McHale. Other methods of heating such as uniform induction coils, electrical resistance elements and indirect gas firing may also be utilized and are disclosed in U.S. Pat. Nos. 3,012,861 to Ling and 3,012,862 to Bertrand et al.
The decomposition of silane or a halosilane in a heated fluidized bed reactor may result in an undesirable deposit of silicon on the wall thereof or on other internal components of the fluidized bed. In some instances, as when the wall temperature in the reaction zone exceeds the temperature of the silicon seed particles, the deposition of silicon on the reactor apparatus may occur in preference to the desired deposition of silicon on the fluidized seed particles. Silane or halosilane decomposition in heated fluidized beds may also result in the homogenous decomposition of silane which produces fine silicon powder or dust. This light powder is undesirable in fluidized bed operations as it requires costly additional handling for recovery and consolidation for melting, which handling increases the risk of contaminating the ultra-pure product silicon and results in some material loss.
The need exists, therefore, for the development of an improved heated fluidized bed reactor for the production of high purity silicon for semiconductor and solar cell applications and an improved process for reducing silanes and halosilanes in a fluidized bed reactor without an accumulation of silicon on the internal surfaces therein and without the homogenous formation of silicon dust.
Hence, it is an object of this invention to provide an apparatus and a process for the enhanced heterogeneous formation of high purity silicon.
It is another object of this invention to provide an improved heated fluidized bed reactor whose interior surfaces are not preferred for the deposition of silicon thereon.
It is still another object of the invention to provide a process which enhances the heterogeneous decomposition of a silane or halosilane feed gas passed through a heated fluidized bed reaction zone and which effectively eliminates silicon deposition on the interior surfaces of the reactor.
A further object of this invention is to provide an improved process for the fluidized bed decomposition of silane and halosilanes with a minimal formation of silicon dust.
Still another object of the invention is to provide an energy efficient process for the formation of high purity silicon.
The foregoing and additional objects of this invention will become more fully apparent from the following description and accompanying drawings.